32 research outputs found
Stiffening thermal membranes by cutting
Two-dimensional crystalline membranes have recently been realized
experimentally in such systems as graphene and molybdenum disulfide, sparking a
resurgence in interest in their statistical properties. Thermal fluctuations
can significantly affect the effective mechanical properties of properly
thermalized membranes, renormalizing both bending rigidity and elastic moduli
so that in particular they become stiffer to bending than their bare bending
rigidity would suggest. We use molecular dynamics simulations to examine how
the mechanical behavior of thermalized two-dimensional clamped ribbons
(cantilevers) depends on their precise topology and geometry. We find that a
simple slit smooths roughness as measured by the variance of height
fluctuations. This counterintuitive effect may be due to the counter-posed
coupling of the lips of the slit to twist in the intact regions of the ribbon.Comment: 7 page
The quantum phase transition of itinerant helimagnets
We investigate the quantum phase transition of itinerant electrons from a
paramagnet to a state which displays long-period helical structures due to a
Dzyaloshinskii instability of the ferromagnetic state. In particular, we study
how the self-generated effective long-range interaction recently identified in
itinerant quantum ferromagnets is cut-off by the helical ordering. We find that
for a sufficiently strong Dzyaloshinskii instability the helimagnetic quantum
phase transition is of second order with mean-field exponents. In contrast, for
a weak Dzyaloshinskii instability the transition is analogous to that in
itinerant quantum ferromagnets, i.e. it is of first order, as has been observed
in MnSi.Comment: 5 pages RevTe
Dynamics at a smeared phase transition
We investigate the effects of rare regions on the dynamics of Ising magnets
with planar defects, i.e., disorder perfectly correlated in two dimensions. In
these systems, the magnetic phase transition is smeared because static
long-range order can develop on isolated rare regions. We first study an
infinite-range model by numerically solving local dynamic mean-field equations.
Then we use extremal statistics and scaling arguments to discuss the dynamics
beyond mean-field theory. In the tail region of the smeared transition the
dynamics is even slower than in a conventional Griffiths phase: the spin
autocorrelation function decays like a stretched exponential at intermediate
times before approaching the exponentially small equilibrium value following a
power law at late times.Comment: 10 pages, 8eps figures included, final version as publishe
Non-Hookean statistical mechanics of clamped graphene ribbons
Thermally fluctuating sheets and ribbons provide an intriguing forum in which
to investigate strong violations of Hooke's Law: large distance elastic
parameters are in fact not constant, but instead depend on the macroscopic
dimensions. Inspired by recent experiments on free-standing graphene
cantilevers, we combine the statistical mechanics of thin elastic plates and
large-scale numerical simulations to investigate the thermal renormalization of
the bending rigidity of graphene ribbons clamped at one end. For ribbons of
dimensions (with ), the macroscopic bending rigidity
determined from cantilever deformations is independent of the width
when , where is a thermal length scale,
as expected. When , however, this thermally renormalized
bending rigidity begins to systematically increase, in agreement with the
scaling theory, although in our simulations we were not quite able to reach the
system sizes necessary to determine the fully developed power law dependence on
. When the ribbon length , where is the -dependent
thermally renormalized ribbon persistence length, we observe a scaling collapse
and the beginnings of large scale random walk behavior
Order parameter symmetry and mode coupling effects at dirty superconducting quantum phase transitions
We derive an order-parameter field theory for a quantum phase transition
between a disordered metal and an exotic (non-s-wave) superconductor. Mode
coupling effects between the order parameter and other fermionic soft modes
lead to an effective long-range interaction between the anomalous density
fluctuations which is reflected in singularities in the free energy functional.
However, this long-range interaction is not strong enough to suppress disorder
fluctuations. The asymptotic critical region is characterized by run-away flow
to large disorder. For weak coupling, this asymptotic region is very narrow. It
is preempted by a wide crossover regime with mean-field critical behavior and,
in the p-wave case, logarithmic corrections to scaling in all dimensions.Comment: final version as publishe
Anisotropic London Penetration Depth and Superfluid Density in Single Crystals of Iron-based Pnictide Superconductors
In- and out-of-plane magnetic penetration depths were measured in three
iron-based pnictide superconducting systems. All studied samples of both 122
systems show a robust power-law behavior, , with the
sample-dependent exponent n=2-2.5, which is indicative of unconventional
pairing. This scenario could be possible either through scattering in a state or due to nodes in the superconducting gap. In the Nd-1111 system, the
interpretation of data may be obscured by the magnetism of rare-earth ions. The
overall anisotropy of the pnictide superconductors is small. The 1111 system is
about two times more anisotropic than the 122 system. Our data and analysis
suggest that the iron-based pnictides are complex superconductors in which a
multiband three-dimensional electronic structure and strong magnetic
fluctuations play important roles.Comment: submitted to a special issue of Physica C on superconducting
pnictide
Quantum Griffiths effects and smeared phase transitions in metals: theory and experiment
In this paper, we review theoretical and experimental research on rare region
effects at quantum phase transitions in disordered itinerant electron systems.
After summarizing a few basic concepts about phase transitions in the presence
of quenched randomness, we introduce the idea of rare regions and discuss their
importance. We then analyze in detail the different phenomena that can arise at
magnetic quantum phase transitions in disordered metals, including quantum
Griffiths singularities, smeared phase transitions, and cluster-glass
formation. For each scenario, we discuss the resulting phase diagram and
summarize the behavior of various observables. We then review several recent
experiments that provide examples of these rare region phenomena. We conclude
by discussing limitations of current approaches and open questions.Comment: 31 pages, 7 eps figures included, v2: discussion of the dissipative
Ising chain fixed, references added, v3: final version as publishe
Near-degeneracy of several pairing channels in multiorbital models for the Fe-pnictides
Weak-coupling approaches to the pairing problem in the iron pnictide
superconductors have predicted a wide variety of superconducting ground states.
We argue here that this is due both to the inadequacy of certain approximations
to the effective low-energy band structure, and to the natural near-degeneracy
of different pairing channels in superconductors with many distinct Fermi
surface sheets. In particular, we review attempts to construct two-orbital
effective band models, the argument for their fundamental inconsistency with
the symmetry of these materials, and the comparison of the dynamical
susceptibilities in two- and five-orbital models. We then present results for
the magnetic properties, pairing interactions, and pairing instabilities within
a five-orbital Random Phase Approximation model. We discuss the robustness of
these results for different dopings, interaction strengths, and variations in
band structure. Within the parameter space explored, an anisotropic,
sign-changing s-wave state and a d_x2-y2 state are nearly degenerate, due to
the near nesting of Fermi surface sheets.Comment: 17 pages, 23 figure